Reproduction steps:
1) Download and open "911830_buildpath.zip" project
2) Press "Build/Build for Android" button located in (top) menu
-- Notice that build fails with "Building to the Assets folder is not allowed." error message

Note:
If you write any other directory and add "/../" to it, everything works as expected.
That means, this issue is reproducible only when user tries to build project in Assets' folder parent.

Regression appeared in 5.6.0b1

Reproduced with:
5.6.0b1, 5.6.1p1, 2017.1.0b5

Unable to reproduce with:
5.5.3p3

Reproduced on Mac and Windows with Android, iOS, OSX and Windows platforms as build target platforms